Author/Authors :
Iodice، نويسنده , , M. and Cisbani، نويسنده , , E. and Colilli، نويسنده , , S. and Crateri، نويسنده , , R. and Frullani، نويسنده , , S. and Garibaldi، نويسنده , , F. and Giuliani، نويسنده , , F. and Gricia، نويسنده , , M. and Lucentini، نويسنده , , M. and Mostarda، نويسنده , , A. and Pierangeli، نويسنده , , L. and Santavenere، نويسنده , , F. and Urciuoli، نويسنده , , G.M. and De Leo، نويسنده , , R. and Lagamba، نويسنده , , Zakary L. and Leone، نويسنده , , A. and Perrino، نويسنده , , R. and Kerhoas، نويسنده , , S. and Lugol، نويسنده , , I.C. and Mazeav، نويسنده , , B. and Vernin، نويسنده , , P. and Zaccarian، نويسنده , , A.، نويسنده ,
Abstract :
Two threshold gas Cherenkov counters have been constructed for the electron and hadron High Resolution Spectrometers (HRS) of the Jefferson Lab Experimental Hall-A. These counters are intended to separate electrons/positrons from other particles up to 4 GeVc. The counters are operated at atmospheric pressure with CO2. Each counter is equipped with ten mirrors. Light weight, thin spherical mirrors (∼ 5.5 × 10−3 radiation lengths) have been employed resulting in a total thickness of ∼ 1.4 × 10−2 radiation lengths crossed by the particles. A prototype of the counter has been tested at CERN with a mixed beam of positrons, pions and protons from 1 to 4 GeVc. Its detection efficiency for positrons and the rejection ratios for pions and protons have been measured as a function of the pulse height response (or equivalently the number of photoelectrons). An improvement of 34% in the number of photoelectrons has been obtained by using a wavelength shifter coated on the photocathode glass window. With such an improvement in 1 m long radiator, an inefficiency for positrons less than 10−3 and rejection ratios πe at the level of few 10−3 and pe smaller than 10−3 have been obtained for pulse heights above 2 photoelectrons. Contaminations of particles below the Cherenkov threshold is fully understood considering δ-rays production.
